I don't own one, but one of our league places has two 8' Diamonds with tighter than 4.5" pockets. It is for sure tougher but not horrible to play on. For a decent player it won't make much difference, if you are shooting a lot of shots off the rail that go in now, they are not likely to go in with a tighter Diamond pocket.
There was an A player that was about to play Mike Dechaine on a tight pocket table, he said "it will slow him down a bit, but it will slow me down a lot", so really depends on the player. If it's a bit tighter, say 4.3" or so, would be a nice practice table. I would not go any tighter or it not be much fun to play on.